The determination of directions, angles and lengths as attitudes is required in many fields of use, such as, for example, in geodetic and industrial surveying. Developments in angle measuring technology led through mechanical reading processes to fully automated angle measurement according to the prior art to date.
Known automated attitude measurement apparatuses generally comprise a code carrier and a scanning device. In angle measurement devices, the code carrier is usually formed so as to be rotatable about an axis relative to the scanning device, an angular position of the code carrier then representing the magnitude to be measured. The code carrier may have, for example, a graduation or coding for position determination, it being possible for the coding to be applied to a surface or lateral surface of the code carrier.
For automatic detection of the attitude, the code carrier moveable relative to the scanning device is scanned by means of different techniques. Known scanning methods are electronic-magnetic, electronic and optoelectronic methods. The following statements relate to optoelectronic scanning methods and scanning devices which have in particular an illumination device and a detector.
For determining, for example, angular positions from 0° to 360°, the coding is usually arranged in a full circle. The angular resolution of the full circle is determined by the type of coding and by the scanning device used for reading the coding. Thus, the angular resolution is increased, for example, by applying a code in a plurality of tracks or by a finer graduation, the achievable resolution being limited for manufacturing and cost reasons. For example, arrangements of one or more detectors are known for reading of the code. CCD line arrays or CCD area arrays may represent, for example, such detectors.
Swiss patent CH 658514 A5 discloses such an apparatus for measuring an angular position. There, a mark whose position relative to a surface of sensors represents the quantity to be measured is focused onto this surface. The output signals of the sensors are passed into an evaluation circuit which sequentially determines the distribution of the intensity of the signals generated by the sensors. The position of the mark in relation to the surface of sensors can be derived from the intensity distribution.
The dimensions of an attitude measurement device for geodetic devices are advantageously to be kept small. To permit a correspondingly compact design which is not very complicated, the illumination device and the detector of the attitude measurement device have for some time been arranged on a common circuit board having an electrical supply, and not as in the past above and below a code carrier, in each case on a separate circuit board having an electrical supply. In the case of attitude measurement devices of the prior art having a side-by-side arrangement of the detector and of the illumination device, the emitted beams are deflected, for example, by a deflection element having two planar, reflective surfaces, so that an image of the code is produced on the detector by the code carrier arranged downstream in the beam path. Optionally, the emitted beams can be collimated by means of an optical system arranged directly downstream of the illumination source.
For example, U.S. Pat. No. 7,145,127 P2 describes a deflection element for an attitude measurement device, where the deflection element has two surfaces oriented towards one another and intended for double reflection of the emitted beams and carries a code focusable onto a sensor and is thus additionally formed as a code carrier.
For achieving as homogeneous an illumination of the code carrier as possible and for producing as uniform a code projection as possible on the detector, laser diodes are now increasingly being used as radiation sources for producing the code projection. For example, EP 0643286 discloses an optoelectronic angle measurement device comprising a semiconductor laser diode as a radiation source. To date, the diodes have been arranged in such a way that the emitter edge of the laser diodes is aligned parallel to the sensor array detecting the code projection. Since the emitted radiation has a higher divergence in the direction transverse to the emission edge than in the direction parallel to the emission edge, downstream arrangement of a beam shaping optical system for collimation of the radiation in a plane perpendicular to the emitter edge is known.
The achievable accuracy of such an attitude measurement depends on a plurality of factors, such as, for example, on the contrast of the code shadow projection produced on the sensor array.
Furthermore, in the case of a code carrier moved relative to the detector, the shadow projection of the code onto the detector may be correspondingly blurred, depending on the exposure time. In order to achieve approximately an accuracy of the relative attitude of the code carrier to the detector of 0.05 μm or, in the determination of a rotational position of the code carrier, of 0.25 arc second, the exposure time must not exceed a duration of about 200 ns. In the case of measuring devices of the prior art, however, the quality of the code projection produced on the detector would generally not be sufficient in an exposure time of only 200 ns to reliably read the position code.